It is now possible to create near-atomically flat silicon, which could soon lead to a new base for new types of biological and chemical sensors.
“In essence, we have made perfect silicon surfaces in a beaker,” said team leader Melissa Hines, a chemist at Cornell University. Researchers had made perfectly flat silicon before, but the prior work focused on silicon surfaces cut along a plane of the crystal not used in the electronics industry. Hines’ team has created the flat surfaces along the industry-standard crystal orientation.
The creation of the team’s first near-atomically flat surface came as a bit of a surprise. At first the team thought the dissolving process they used to clean the silicon left rough, bumpy surfaces. Hines asked one of her graduate students to take an picture of the surface using an instrument called a scanning tunneling microscope (STM) that can image surfaces to atomic-level detail.
“When we looked at the surface, we unexpectedly realized, ‘Hey, this actually looks very flat,’” Hines said.
The microscope images showed a surface with alternating single-atom-wide rows. Using the additional tools of computer simulations and infrared spectroscopy the researchers determined the silicon atoms in the rows bonded to hydrogen atoms that acted like a wax, preventing the surface from further reacting once it was set out in the air.
“What that means is that if you take this perfectly flat surface, pull it out of the aqueous reactants, and rinse it off, you can leave it lying around in room air on the order of 10-20 minutes without it starting to react,” Hines said. “If you had told me as a graduate student that you could have a clean surface that could just hang out in air for 10 minutes, I would have thought you were crazy.”
The team believes that part of the reason their silicon surfaces are so flat is that they dip the wafers in and out of solution approximately every 15 seconds, preventing bubbles from the reaction from building up and causing uneven etching. However, they also credit the STM images for helping them to realize just how flat the surfaces were.
The team built off the information from the images by using computer simulations and other tools to reveal the exact chemical reaction steps that took place in solution. “Experimentally, this is very simple experiment: you take a piece of silicon, you swirl it in a beaker with solution, and then you pull it out and look at it. To be honest, there is no reason to think that Bell Labs did not make a surface as good as ours twenty years ago, but they did not look at it with STM, so they did not know,” Hines said.
Hines’ team is now working on adding molecules to the atomically smooth, hydrogen-terminated silicon surface in the hopes of building new chemical or biological sensors.
“At this point, I can’t tell you exactly how we will accomplish this, but we have promising results and hope to be able to report more soon,” she said.